In recent years, pre-expanded beads of not a crosslinked polyethylene resin but a noncrosslinked polyethylene resin are being developed because of advantages thereof that some steps including the step of crosslinking are unnecessary and recycling is possible (see, for example, JP-B-60-10047, JP-B-60-10048, JP-A-6-316645, JP-A-6-157803, etc.). (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application", respectively.) These techniques are intended to eliminate the drawback in expansion molding using a polyethylene resin as a feedstock resin wherein when the resin has not been modified by crosslinking, pre-expanded beads of satisfactory quality cannot be obtained therefrom and a foamed molding suitable for practical use cannot hence be obtained, and to enable pre-expanded beads and foamed moldings of satisfactory quality to be obtained through the foaming of a noncrosslinked polyethylene resin.
Difficulties in the foaming of polyethylene resins are attributable to the fact that a considerable proportion of the foaming agent (inorganic gas) infiltrated into beads escapes during heating for foaming, thereby making it difficult to obtain expanded beads having an expected cell structure and expansion ratio, and that the resins have a large temperature dependence of melt viscosity, thereby making it difficult to regulate the melt viscosity so as to be suitable for foaming. In this connection, the temperature range in which noncrosslinked high-pressure-process low-density polyethylene resins can retain a melt viscosity suitable for foaming is said to be 1.degree. C. at the most. Because of the difficult in controling the temperature, optimal foaming is difficult. For example, JP-A-6-316645 discloses pre-expanded beads obtained using as a base resin a noncrosslinked ethylene resin that has a specific melt viscosity range so as to impart excellent in-mold moldability to the pre-expanded beads, and also discloses, as an example of the base resin having such specific viscosity properties, a mixed resin comprising a linear low-density polyethylene having a resin density of from 0.920 to 0.940 g/cm.sup.3 and a high-density polyethylene having a resin density of 0.940 g/cm.sup.3 or higher. Further, JP-A-6-157803 discloses noncrosslinked pre-expanded beads made of a mixed resin, as a base resin, comprising from 20 to 85 wt % low-density polyethylene, from 0 to 45 wt % linear low-density polyethylene, and from 0 to 40 wt % high-density polyethylene and having a resin density of from 0.920 to 0.940 g/cm.sup.3 and also discloses a process for producing the same, in order to provide molded foams equal to molded foams of crosslinked polyethylene resins in flexibility, toughness, and durability in compression strain and to provide pre-expanded beads of a noncrosslinked polyethylene resin which are excellent in the ability to expand during in-mold molding and in fusibility.
There is a recent trend in the market wherein foamed moldings are having more complicated three-dimensional shapes. For example, FIG. 1 shows an example of foamed moldings of complicated shapes which are highly desired in the market; the example shown is a set of cushioning foamed moldings for use in the assembly packaging of liquid-crystal modules. FIG. 1 (A) and (B) show foamed moldings of the same shape, which are put together in such a manner that a liquid-crystal module (D) is sandwiched therebetween and are then put in a corrugated fiberboard container (C). The frame 1 and the partitions 2 in FIG. 1 (B) have been formed according to the shape of the liquid-crystal module (D) in order to firmly hold the module, while the projections 3 to 5 in FIG. 1 (A) and the projections 6 and 7 in FIG. 1 (B) have been disposed for the purpose of cushioning the impact as a result of falling, etc. and are shaped in a manner which is less apt to break or damage. Since users of cushioning assembly packaging materials desire to maximize the number of articles to be packed per package in order to reduce transportation cost, a strategy for increasing the capacity of the packaging material is to reduce the thickness of the frame and partitions as much as possible while still preventing damage to the contents. In the example shown, the frame 1 and the partitions 2 have thin-wall parts having a thickness as small as about 10 mm. As a result of market demand to reduce the amount of expanded beads per unit and to reduce the transportation cost by heightening the efficiency of packaging as described above, foamed moldings (especially for use as cushioning materials) generally have complicated shapes having many thick and thin parts and many recessed and projected parts.
Another important objective is to further increase the strength (rigidity) of foamed moldings. In cushioning material applications, foamed moldings having a higher strength can have a reduced contents-supporting area while still maintaining the required cushioning performance, and reducing the wall thickness. In addition, such higher-strength foamed moldings can lower the weight per unit (by a reduction in bulk density) which has not been attained with any conventional pre-expanded beads, whereby a further reduction in the amount of expanded beads per unit is possible.
Under these circumstances, moldings having many recessed and projected parts are desired to have a higher level of appearance, i.e., greater in-mold moldability, and a greater level of compression strength than are currently available.
However, it is extremely difficult to produce a foamed molding of a complicated shape such as that shown in FIG. 1, which achieves the desired performance, by in-mold hot molding from any of the conventional pre-expanded noncrosslinked polyethylene beads including those disclosed in the references cited above. The difficulty in using a mold of a complicated shape having many recessed and projected parts and many parts for forming thick and thin walls, etc., is the thin-wall parts of the expandable beads. (e.g., the frame 1 in FIG. 1, etc., are apt to be overheated during the heat treatment in the in-mold molding because of their good permeability to steam, while thick-wall parts (projected parts, e.g., projections with a height of about 50 mm such as those shown by numerals 3 to 5 in FIG. 1 (A) and by 6 and 7 in FIG. 1 (B)). Are less apt to be thoroughly because of their low permeability to steam, and parts like the partitions 2 are apt to develop hot spots, for example, because steam holes become disposed therearound face to face at a small distance and interference among steam streams occurs to inhibit temperature rising. If the heating temperature for the molding is adjusted to a value at which the parts likely to suffer overheating neither shrink nor deteriorate, the resulting molding partly has insufficient fusion bonding between beads. Thus, heating spots tend to be formed. If the heating temperature for the molding is adjusted to a value suitable for the parts less apt to be heated, a defective molding results which has partly undergone cell breakage and shrinkage.
The cell breakage and shrinkage phenomena are attributable to properties of the pre-expanded noncrosslinked polyethylene resin beads, that is, a small difference between the temperature at which the expanded beads begin to fuse and the temperature at which the expanded beads begin to undergo cell breakage and shrinkage (a narrow range of heating temperatures suitable for molding). In addition, in actual long-term production, the steam molding pressure always fluctuates due to fluctuations of pressure of the steam source, etc., so that molding cannot be performed at a constant heating temperature. Therefore, in order to attain stable production with a low percentage of rejects daring a long-term production operation, it is necessary to widen the range of heating temperatures suitable for molding upon comparison with conventional compositions.
In order to highly expand pre-expanded beads to obtain a high-strength low-bulk-density foamed molding which retains intact high compression strength and rigidity, which is the other desire of the market, the base resin should have a density not lower than 0.940 g/cm.sup.3. However, in conventional noncrosslinked pre-expanded beads, higher densities of the base resin tend to result in a narrower range of heating temperatures suitable for molding, and the moldings having complicated shapes cannot be obtained from just any base resin having a density of 0.940 g/cm.sup.3.
Conventionally, there are two processes for producing pre-expanded beads. One process is the so-called "flash expansion method," which comprises introducing an aqueous suspension of expandable resin beads impregnated with a foaming agent into a pressure vessel, regulating the expandable resin beads in the vessel so as to have a temperature suitable for foaming, and then discharging the expandable resin beads from one end of the vessel together with the aqueous suspension medium into an atmosphere having ordinary temperature and ordinary pressure to instantaneously expand the expandable resin beads, whereby expanded beads having a desired high expansion ratio are obtained in one step. The other process is the so-called "multistage temperature-rising expansion method," which comprises infiltrating a small amount of a foaming agent into resin beads in a pressure vessel, taking out the impregnated resin beads after cooling, transferring the same to a foaming tank, heating the foaming tank to a temperature suitable for foaming to expand the resin beads to thereby obtain expanded beads having a low expansion ratio, and then subjecting the obtained expanded beads several times to a step in which an inorganic gas is forced into cells of the expanded beads in a pressure vessel to give expandable expanded beads and the expandable beads are expanded by heating to obtain expanded beads having a higher expansion ratio, whereby the expansion ratio of expanded beads is heightened stepwise. These two expansion methods were developed for polyethylene resins which have proven to be difficult to expand.
However, when the "flash expansion method" is compared with the "multistage temperature-rising expansion method" with regards to the inhibition of the flying of the foaming agent and the degree of control of the range of temperatures suitable for foaming with heating, the "flush expansion method" is more advantageous. The reasons for this are as follows. First, in the "multistage temperature-rising expansion method," the step in which expandable beads to be foamed (expanded) are heated to a temperature suitable for foaming is conducted in an open foaming tank (vessel). Because of this, the foaming agent (inorganic gas) cannot be inhibited from going away during this heating and, in addition, the method undergoes a phenomenon in which foaming (expansion) begins before the expandable beads are heated to a temperature suitable for foaming. Consequently, even though the multistage temperature-rising expansion method is applicable to resins (e.g., resins modified by crosslinking) which have a sufficiently widened range of temperatures suitable for foaming with heating (temperatures at which the viscosity is suitable for foaming), the method is unsuitable for linear high-density polyethylene resins having a narrow range of temperatures suitable for foaming because such polyethylene resins undergo a considerable disorder in cell structure during expansion, making it very difficult to obtain expanded beads from the noncrosslinked polyethylene resins. It is noted in this connection that when linear low-density polyethylene resins are used, expanded beads may be obtained therefrom by the "multistage temperature-rising expansion method" because such resins have a slightly wider range of temperatures suitable for foaming due to the function of a comonomer component. However, in this case also, these expanded beads have a disorder in cell structure caused during expansion and this disorder in cell structure prevents these pre-expanded beads from giving a molding of good quality.
In contrast, the "flash expansion method" is advantageous in that since the expandable resin beads in a closed vessel can be kept in a state suitable for foaming until discharge and expansion by regulating or controlling the pressure and temperature of the vessel. Even a resin having a relatively narrow range of temperatures suitable for foaming (viscosities suitable for foaming) can be easily expanded. As a result, pre-expanded beads having an expansion ratio as high as 60 cc/g can be obtained through one-step expansion operation.
In view of the fact that the current "multistage temperature-rising expansion method" cannot be used for obtaining expanded beads from any noncrosslinked polyethylene resin having a high density (not lower than 0.936 g/cm.sup.3, especially not lower than 0.940 g/cm.sup.3), it has been extremely difficult to produce expanded beads which have a wide temperature range suitable for molding as well as satisfactory basic properties.
Accordingly, an object of the present invention is to provide pre-expanded noncrosslinked polyethylene resin beads which have a wide temperature range usable in in-mold molding for producing a satisfactory molding of a complicated shape, and which can give a molding having a desired compression strength and excellent cushioning properties and still have a high expansion ratio.
Another object of the present invention is to provide a process by which expanded noncrosslinked polyethylene resin beads having the properties described above can be provided easily.
Still another object of the present invention is to provide a "multistage temperature-rising expansion method" by which expanded noncrosslinked polyethylene resin beads having the properties described above can be easily provided.